Constant HDD activity

Good day, I'd like to ask for your help concerning a problem with my ISPConfig 3 server.

It's a "perfect setup"-based 64-bit CentOS 5.7 server.

For a long time everything was perfect. Then yesterday I noticed that the server started to operate very slowly. I mean, VERY SLOWLY - a few minutes for a simple web page to load, having to wait a lot for the login/password prompt while connecting by ssh, etc.

I also noticed that the HDD led was showing constant disc activity.

I rebooted the server, and the booting process looked normal (fast, no errors), but shortly afterwards (1-3 minutes) this unusual hard disk activity was back, and the server was again slow.

I "yum updated" to the latest versions of the software packages, and I also updated to the latest version of ISPConfig. (I have to say that I had to do "yum update" step by step, package by package, because, as I said, I had about 2 minutes of normal server operation upon reboot, after which even yum update took ages.

ISPConfig update went smoothly, without errors.

Now I have all the latest software, but the problem is still here. Like I said, there are no errors during the booting process - and it's as fast as it was earlier. Also, the server is fairly new, with 8 gigs of RAM and a lot of free hard disc space.

I wonder what could be causing this HDD activity? Is there a way to monitor it?

Thank you very much for your reply, I'll definitely explore the "iotop" tool.

At the moment, however, I think that I have found the source of the problem.

One of my client's sites was an old SMF forum. It was a neglected forum, probably hacked, and so a target for malicious scripts. I explored the logs and found out that these scripts posted dozens of messages per second.

I disabled this forum and - at least for now - the server operation is normal. I hope that this was indeed the cause of the problem.